Epoxy resin compositions have been designed and used as dielectrics for wires, coils, windings, diodes, resistors, capacitors, and the like, including use as encapsulating compounds for such electrical and electronic devices or apparatus. In the manufacture of such devices protected by an encapsulant, the epoxy composition in the uncured state is applied to the device, and then cured or hardened to provide a relatively strong, infusible, cross-linked polymer. These epoxy resin compositions, when cured or thermoset, provide coatings exhibiting good electrical and physical properties, such as relatively high dielectric strength and volume resistivity, and high tensile, compressive and flexural strengths. The epoxy compounds, however, exhibit certain disadvantages or deficiencies, most notably a relatively high dissipation factor at operating temperatures which, for most such devices ranges from about 100.degree. to 125.degree. C. or slightly higher.
The dissipation factor can be determined by calculating the capacitance and conductance of a dielectric material, which are two fundamental electrical characteristics of such materials. When an alternating electrical field is created within a capacitor, the oscillation is produced at -he same frequency but with a phase angle shift (delta). The phase angle shift is measured by comparing the applied voltage to the measured current, for example in a high voltage coil or sweep transformer which has distributed capacity. The tangent of delta is the dissipation factor and represents the resistive element for the dielectric material. These electrical losses are manifested by a temperature rise. The higher the dissipation factor the greater the heat buildup. As the temperature increases, the dissipation factor may increase causing still more increase in temperature. At that point, the device may become involved in a thermal runaway and consequently self-destruct.
The epoxy compositions known and used in the electrical insulating art, e. g. encapsulating art, typically comprise an epoxy resin, and a curing agent or hardener. Depending on the end use and properties sought for the end product, the epoxy may be powder or liquid at room temperature, and very typically such as for bisphenol-A based resins have an epoxide equivalent weight ranging from about 170 to 750. The curing agents most commonly used for these epoxy resins include the amine type agents such as aliphatic polyamides which permit cross-linking at about room temperature, and acid anhydride type agents such as maleic anhydride or phthalic anhydride which require cross-linking at elevated temperatures. Also, it is known to incorporate small amounts of a catalyst or accelerator such as imidazole or imidazole derivatives, tertiary amines or stannous salts. Numerous fillers, such as silica, hydrated alumina, calcium carbonate, magnesium silicate, aluminum silicate, antimony oxide or zirconium silicate, have been added to the resin compounds to improve such properties as dielectric strength, mechanical strength, and flame retardancy, as well as provide for cost reduction.
However, known epoxy compositions, in addition to the disadvantages noted above, during handling are sensitive to ratio changes between the epoxy resin and the hardener, and frequently possess a short shelf life. Certain of these deficiencies have been overcome by one or more techniques shown in the art. For example, U.S. Pat. No. 3,909,408 to Ogata et al discloses admixing with the resin an organo-tetra-substituted borate of imidazole or imidazole derivative in order to improve storage stability yet provide rapid curing on heating. The resin composition also may include a small amount of a curing catalyst such as an acid anhydride, as well as fillers or colorants, depending on the end use, and mixing is accomplished by applying a shearing force such as by an extruder, ribbon mixer, or the like. High shear for mixing epoxy powder and liquid catalyst is disclosed in U.S. Pat. No. 4,558,077 to Gray, for use in making a bonding agent for magnets. In order to prolong the pot life or working or handling time, U.S. Pat. No. 4,009,141 to Nichols et al discloses a two component system, one of which is a cycloaliphatic epoxy resin and filler and the other is the anhydride hardener and filler, and the two components are admixed just prior to the molding operation.
In the manufacture of the epoxy compounds, any marked sensitivity to ratio changes between resin and hardener is still a deficiency because upsets occur during production, and equally important there is the need to lower the dissipation factor of the cured product while still controlling or lowering costs. It is the purpose of this invention to provide for these advantages, as well as to overcome the other deficiencies of the prior art. These together with other objects and advantages will best be understood by referring to the following description of the invention, and to the accompanying drawings.